Quasi-Steady High-Pressure Droplet Model for Diesel Sprays

Droplet vaporization models that are currently employed in simulating diesel engine sprays are based on a quasi-steady, low-pressure formulation. This formulation does not adequately represent many high-pressure effects, such as non-ideal gas behavior, solubility of gases into liquid, pressure dependence of liquid- and gas-phase thermophysical properties, and transient liquid transport in the droplet interior. More importantly, the quasi-steady assumption becomes increasingly questionable as the ambient pressure approaches and /or exceeds the fuel critical pressure. In the present study, a high-pressure, quasi-steady vaporization model is developed. Except for the quasi-steady assumption that is retained in the model, it incorporates all the other high-pressure effects. The applicability of this model for predicting droplet vaporization under diesel-like environment is evaluated by comparing its predictions with those from a more comprehensive transient model, which is first validated by using the available experimental data. Results indicate a fairly good agreement between the quasi-steady and transient models for pressure upto the fuel critical pressure. At pressures above this value, the quasi-steady model underpredicts the vaporization rate compared to that predicted by the transient model, and the amount of underprediction becomes increasingly more significant at higher pressures. The differences can be attributed to the quasi-steady gas-phase assumption that leads to larger gas film thickness and, therefore, reduced gas-phase heat and mass fluxes at the droplet surface for the quasi-steady model. Another factor contributing to the reduced vaporization rate for the quasi-steady model is the relatively larger droplet heatup time.